Conventionally, two-stroke engines have been used as drive engines for working machines to mow plans, such as a portable brush cutter or trimmer carried by hand and a backpack working machine carried on the back of the user. However, due to increasing public awareness regarding environmental issues, enhancement of emission control and so forth, there is an increasing demand to substitute four-stroke engines for two-stroke engines, as driving sources.
Such a four-stroke engine tends to be heavy because of having greater number of necessary parts than of a two-stroke engine. However, in a case of a portable working machine, the user works, naturally carrying the portable working machine. Therefore, there is a demand for a light-weight four-stroke engine.
Here, a lubrication apparatus for a four-stroke engine has been proposed, which circulates oil using pressure fluctuation in a crank chamber without providing a separate pump for lubrication (see Patent Literature 1). This lubrication apparatus has a first oil passageway configured to penetrate a crankshaft to allow communication between an oil tank and a crank chamber, and lubricates around the crankshaft by supplying oil mist produced in the oil tank to the crank chamber via the first oil passageway, using that a negative pressure is created in the crank chamber. In addition, using that a positive pressure is created in the crank chamber, oil mist produced and scattering in the oil tank is supplied to lubricate a power transmission mechanism (including an intake valve and an exhaust valve) in a first valve operating chamber and a cam mechanism in a second valve operating chamber, which are arranged above the oil tank when the engine stands upright.
This lubrication apparatus is configured to suck the oil accumulated in the valve operating chamber via a siphon and return the oil to the crank chamber, using that a negative pressure is created in the crank chamber as a piston moves due to the rotation of the crankshaft.